Spatial differentiation and trade-off/synergy relationship of soil fertility in typical karst peak-cluster depressions

MA Guobin; QIN Xingming; HU Baoqing; QU Zihan; SUN Qi

doi:10.11932/karst2025y005

Abstract：A scientific evaluation of soil fertility levels in karst peak-cluster depressions at a grid scale is essential for implementing precise fertilization and conservation strategies at the regional level.To provide targeted support for the effective management and cultivation of soil fertility in these depressions,this study selected typical peak-cluster depressions in Pingguo City,Guangxi,as the research object.A 500 m×500 m grid was employed as the evaluation unit,and soil data were collected through a combination of field sampling and Kriging interpolation.An index system was developed,incorporating pH,organic matter(OM),total nitrogen(TN),total phosphorus(TP),total potassium(TK)and land use types.The Analytic Hierarchy Process(AHP)was utilized to determine the weights of these indicators,and a multi-factor comprehensive evaluation method was applied to assess the overall soil fertility.Furthermore,semi-variance functions,spatial clustering models(including global and local Moran's Ⅰ),and Spearman' s correlation coefficients were employed to systematically analyze the spatial differentiation characteristics of soil fertility and the trade-off/synergy relationships among the indicators. The results indicate that:(1)spatial differentiation is significant:TN,TP and OM contents generally exhibit a decreasing pattern from the northeast to the southwest,with weak spatial correlation(nugget coefficient＞75％).This pattern is mainly influenced by anthropogenic stochastic activities,such as fertilizer application and planting methods.In contrast,pH value demonstrates the strongest spatial correlation with a nugget coefficient of 17％,which is predominantly governed by the structural factors of the karst geological background.Additionally,the spatial distribution of TK is largely opposite to that of TN,TP and OM.(2)The overall soil fertility is characterized by a relatively deficient level,with an average value of 2.93.Grid cells classified as moderate(Grade 3),relatively deficient(Grade 4),and deficient(Grade 5)account for 86.88％of the total area.Spatially,soil fertility exhibits a differentiation pattern of being higher in the northeast and lower in the southwest.High-fertility zones are concentrated in low-slope forestlands and agricultural lands in the northeastern region,while low-fertility zones are found in sloping farmlands,construction sites,and steep-slope forestlands in the northwestern area.(3)Significant trade-off/synergistic effects exist among indicators:TP,TN and OM show significant synergy effects in enhancing comprehensive fertility(Spearman's correlation coefficient＞0.389),with contribution rankings as TP＞TN＞OM.However,TK exhibits a significant trade-off relationship(negative correlation)with the other four indicators(TN,TP,OM and pH),particularly showing the strongest trade-off effect with TN.Spatial agglomeration analysis indicates that OM,TN,TP,and TK exhibit strong spatial autocorrelation(Moran's I＞0.47),with predominant agglomeration type being "low-low" synergy-weakening patterns,while "high-high" synergy-enhancing patterns are mainly distributed in the northeastern agricultural land area.The relationship between land use and spatial synergy/trade-off effects of OM and TN is generally moderate. In conclusion,soil fertility in typical karst peak-cluster depressions is relatively low and exhibits significant spatial heterogeneity,with complex trade-offs and synergies among nutrients.Accordingly,it is proposed that differentiated measures targeting different fertility grades and spatial areas should be implemented,including soil and water conservation,optimization of fertilization structures(with a focus on phosphorus and nitrogen synergy supplementation,as well as potassium balance),and adjustments to land-use planning to achieve effective enhancement and sustainable management of soil fertility.